I was having a problem understanding what "run at" meant but more re-reading of Steve's stuff sort of straightened me out. The key was the 2.5 x carrier level. This started making sense when I divided the HV by 2.5 to learn what that carrier level will be. The carrier level on the PDM Generator now makes sense. This is a difficult concept for an old tube buzzard to get a handle on.

steve_qix wrote:The filter is function of the load resistance presented to it, in this case, by the RF amplifier. It doesn't matter about the input voltage, power supply voltage or any other parameter.

But how do you determine the load resistance without knowing the voltage and current?

>I use a butterworth calculator, and plug in the number of poles (always inductor input), and the terminating resistance (the actual DC resistance of the rf amplifier(s) ).

Glad you mentioned that last part as I was reading your schematic backwards and it looked like C2 was the first pole.

Another confusion (several); your article talks about 6 poles but the schematic only shows 4.

When I tried plugging your numbers into my filter calculator, it would not allow 4 poles. 3 or 5 but not 4. I went back to the Pronine web site and found the one in my last posting which allows 4 poles and corroborates your numbers exactly so I was able to use it to come up with numbers for any value of R. I don't know what that other calculator was but I learned something in the process.

After doing this I sort of lost interest in the air core inductors. I have two FT114-43 cores. Do you think they would handle 50W carrier level modulation?

I don't know if the FT114-43 will work or not in this application. The biggest problem you will have to deal with is core saturation. Magnetics has a core calculator that works very well, and you can use this to see if the core will saturate or not. It also fill figure out how many turns you need, etc.

One of the major drawbacks of using non-air inductors is the change in inducatance VS current. To this end, I have usually speced HI FLUX, MEGAFLUX or X-FLUX inductors. These are some of the most stable materials available.

So, when using a non-air core inductor(s), be SURE to calculate the inductance variation (the Magnetics calculator will do this) based on the absolutest highest positive modulation that the modulator will deliver. I usually tripple the DC current (that's 200% positive) and do all the calculations based on that number. So, if my DC current is 8 amperes, I do my calculations based on 24 amperes. You probably get away with 2.5x the DC - 150% positive, but definitely NOT the unmodulated DC. If the inductance changes a lot with current, or if there is saturation (the inductance will change a LOT under saturation), your filter effectiveness will change with modulation, and you will likely produce spurious products at the switching frequency, depending on the positive swing at the time.

Filter poles - I use 4 poles with lower power transmitters (400 watts and down), and 6 poles for anything higher power. No particular reason for choosing 400 watts as the threshold.

steve_qix wrote:I don't know if the FT114-43 will work or not in this application. The biggest problem you will have to deal with is core saturation. Magnetics has a core calculator that works very well, and you can use this to see if the core will saturate or not. It also fill figure out how many turns you need, etc.

Try adding 1 turn to the secondary. I just did that and the R went down to 5.5 Ohms, power and current went up and efficiency stayed about the same.

As a point of interest, I started by removing a half turn and it went all to hell. A full turn and it came up somewhere between hell and where I started. I then added a half turn and got nothing useful again and then adding the full turn made it all nice.

Must be some bino core law I do not know. Even though one can have half turns, they do strange things.

My Elmer worked up a Spice model for this RF deck that is fun to play with but we can not find the exact model for the 11n90 and he uses something close.

If you or anyone else is interested, I would be happy to send the model. It runs on LTspice and I don't know if .asc is universal or unique to this program.